Wireless communication devices such as mobile telephones, personal digital assistants (PDAs), notebook and netbook computers, and the like continue to be popular among consumers, thereby driving the need for more reliable services, expanded areas of coverage, and increased functionality. A wireless communication device includes such terms as, for example, a mobile station, a subscriber station, an access terminal, a remote station, a user terminal, a terminal, a subscriber unit, user entity and user equipment, hereinafter referred to as stations (STAs).
A wireless communication system may provide communication for a number of cells, each of which may be serviced by a base station. A base station may be a fixed station that communicates with mobile stations. A base station may alternatively be referred to as an access point (AP).
A subscriber station may communicate with one or more base stations via transmissions on the uplink and the downlink. The uplink (or reverse link) refers to the communication link from the subscriber station to the base station, and the downlink (or forward link) refers to the communication link from the base station to the subscriber station. A wireless communication system may simultaneously support communication for multiple subscriber stations.
Wireless communication systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, and spatial division multiple access (SDMA).
One faster version of IEEE 802.11 is referred to as VHT (Very High Throughput). VHT allows for multiple transmissions to occur in parallel without causing a collision, like SDMA and OFDMA.
A multiple-input-multiple-output (MIMO) wireless network in an SDMA network is a communication system that generally includes an access point (AP) having multiple antennas communicatively coupled to multiple receiving stations. To support a downlink multi-user MIMO network, the access point (AP) may need to schedule multiple packets destined for the multiple stations for simultaneous transmission.
Specifically, the packets can be in accordance with Physical Layer Convergence Procedure (PLCP). The PLCP takes each 802.11 frame that a STA wishes to transmit and forms what the 802.11 standard refers to as a PLCP protocol data unit (PPDU). The resulting PPDU contains a PLCP preamble, PLCP header and PSDU.
If uplink (UL) SDMA is not supported, STAs may contend to transmit acknowledgements. However, the cost of collisions is very high and some STAs outside of an SDMA group may win the contention and start transmitting data frames. To avoid this situation, the AP needs to schedule acknowledgements from different STAs, meaning different STAs should only send back acknowledgements according to the AP's schedule. There is a plurality of techniques for scheduling the response frame. One method requires the use of a medium access control (MAC) duration field to schedule sequential responses. However, the Duration field in the MAC header is traditionally used to provide NAV (Network Allocation Vector) protection for providing transmission opportunity (TXOP) and/or reverse direction grant (RD) techniques. Thus, there is a need for methods and systems that improve MIMO transmissions with reduced collision risk.
In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present specification. Reference characters denote like elements throughout Figures and text.